The present invention relates to a recording medium, a cationic silica dispersion and coating composition useful in the preparation thereof, and methods of preparing them.
A surface coating is sometimes applied to a recording medium in order to improve its printing properties. For example, a glossy coating can impart a superior feel and a photograph-like quality to a printed image; a highly absorptive coating can reduce the smearing and rub off of an image; and a coating that immobilizes (i.e., adsorbs) ink dyes at the outer surface of the coating enhances the waterfastness and color density of a printed image.
It remains a challenge to prepare a single coating for a recording medium that is glossy and yet has good absorptive and immobilizing properties. Gloss and dye immobilization can sometimes be achieved by incorporating different types of polymeric resins into a coating. For example, a polyolefin resin, polyester resin, polyamide resin, and/or polycarbonate resin can be used to produce glossiness, while a cationic polymer resin can be used to promote the surface immobilization of an anionic dye. However, inks applied to resin-coated recording media dry relatively slowly, and often have an undesirable tendency to smear and rub off. While some pigments such as certain treated kaolin clays or treated calcium carbonates can immobilize dyes, the overall absorptivity and rate of absorption are sometimes compromised with such pigments.
A need therefore exists for a recording medium having a coating that is glossy and also has good absorptive and immobilizing properties. A need also exists for a method of preparing such a recording medium, and a dispersion and coating composition that can be used in such a method. The present invention provides such a recording medium, method, dispersion, and coating composition.
The recording medium of the present invention comprises a substrate having a glossy coating thereon, wherein the glossy coating comprises cationic silica. The cationic silica comprises silica particles having a mean diameter of less than about 1 xcexcm that have been contacted with an aluminum compound. The cationic silica can be characterized by its colloidal stability, i.e., by the fact that a dispersion thereof in deionized water at a concentration of about 20% by weight and a pH of about 3-4 exhibits a viscosity of less than about 1500 centipoise at relatively low shear rate, e.g., as measured in a Brookfield Model RVT digital viscometer, using spindle no. 5 at 100 RPM, after the dispersion is allowed to stand at a temperature of about 25xc2x0 C. for 180 days immediately following the preparation thereof. The dispersion of the present invention typically has a zeta potential of at least about 50% of the maximum zeta potential in the absence of a substantial excess of aluminum.
The coating of the inventive recording medium has an excellent rate of liquid absorption, a relatively high liquid absorption capacity, and an excellent dye-immobilizing, glossy surface. Images applied to the glossy coating of the inventive recording medium are waterfast, have a high color density, and dry relatively quickly.
The method of preparing the recording medium of the present invention comprises:
(a) providing a substrate,
(b) coating the substrate with a coating formulation comprising the aforementioned cationic silica to provide a coated substrate, and
(c) drying the coated substrate to form the recording medium.
The present invention also provides an aqueous dispersion of cationic silica, wherein:
(a) the cationic silica comprises silica particles that have been contacted with one or more aluminum compounds,
(b) the mean diameter of the silica particles is from about 100 nm to about 1 xcexcm,
(c) the pH of the dispersion is from about 2 to about 6, and
(d) the cationic silica content of the dispersion is from about 5% to about 50% by weight, and the aqueous dispersion preferably is characterized by:
(e) a viscosity of less than about 1500 centipoise at relatively low shear rate after it is allowed to stand at a temperature of about 25xc2x0 C. for 180 days immediately following the preparation thereof, and
(f) a zeta potential of at least about 50% of the maximum zeta potential without the presence of a substantial excess of aluminum.
The present invention further provides a method of preparing the aforementioned aqueous dispersion of cationic silica. The method of the present invention comprises:
(a) providing an aqueous suspension of silica particles, wherein the mean diameter of the particles is less than about 1 xcexcm, and
(b) contacting the silica particles in the suspension with at least one aluminum compound to form a mixture, wherein the zeta potential of the mixture increases rapidly during the initial contacting, but plateaus (i.e., increases slowly) during subsequent contacting, and the contacting is continued until the zeta potential of the mixture plateaus, at which time the contacting is discontinued to provide the aqueous dispersion of cationic silica.
The present invention further provides a coating composition comprising a suitable binder and the aqueous cationic silica dispersion of the present invention.
The inventive method of preparing an aqueous dispersion of cationic silica overcomes at least some of the limitations of previous methods in a number of ways. For example, it is not necessary to rigorously remove electrolytes from the dispersion. Also, dispersion stabilizers such as divalent metals, ammonia, and bicarbonate are not needed. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.